For example, in the fields of railroad control, road control and the like, in order to know the operating conditions of signals and crossings or traffic density on roads, there is used a system in that communication terminals which monitor such conditions and transmit the monitored results are located at respective monitoring spots, and the monitored results are collected from the respective communication terminals (monitoring terminals) and used to control.
A known example of the connection style of such a type of control system is a network, which connects a plurality of communication devices accommodating a single or plurality of local communication terminals through a path according to an ATM (asynchronous transfer mode) switching method to transmit/receive information among the respective local communication terminals through the above path via the corresponding communication device. The individual local communication terminal is located at each monitoring spot to serve as a monitoring terminal to make monitoring and transmit the monitored results.
Besides, development is being made these days on a system which has the above-described network determined as a basic unit and has a connection style to accommodate such a unit network in multiple numbers. FIG. 11 is a schematic diagram showing an example of such a connection style of a conventional communications network system, which consists of a unit network 1 and a unit network 2. In the respective unit networks 1, 2, reference numeral 30 denotes remote communication devices (divided into R1 through R7 and R8 through R14 for convenience of description). And, they are connected in a bus configuration through a path according to, for example, the ATM communication method. And, these remote communication devices 30 accommodate a single or plurality of local communication terminals 40, respectively.
In the aforementioned conventional communications network system, the unit networks 1, 2 independently form a communication connection. And they are generally not connected physically even if they are adjacent to each other. Therefore, if a trouble such as a line breakage occurred in the unit network 1, an alternate path running via the other unit network 2 could not be used to relieve communications between local communication terminals 40 which are adjacent to the troubled spot.
For example, it is assumed that a disconnection fault (marked X) occurs between the remote communication device R3 and the remote communication device R4 as shown in FIG. 12 when the local communication terminal 40 connected to the remote communication device R4 and the local communication terminal 40 connected to the remote communication device R6 are communicating normally in the unit network 1 as shown in FIG. 11. Then, the communication route of the local communication terminal 40 connected to the remote communication device R4 and the local communication terminal 40 connected to the remote communication device R6 is changed as shown FIG. 12, so that communications between the local communication terminal 40 connected to the remote communication device R4 and the local communication terminal 40 connected to the remote communication device R6 can be relieved. However, if the aforementioned trouble (disconnection fault between R3 and R4) occurred while communicating between the local communication terminal 40 connected to the remote communication device R2 and the local communication terminal 40 connected to the remote communication device R5, the communications between the remote communication device R2 and the remote communication device R5 are lost due to the disconnection fault. As a result, the local communication terminal 40 connected to the remote communication device R2 and the local communication terminal 40 connected to the remote communication device R5 cannot communicate to each other.
For the same reason, if the unit network 1 had a broken path between the remote communication device R4 and the remote communication device R5 while communications are being made normally between the local communication terminal 40 connected to the remote communication device R4 and the local communication terminal 40 connected to the remote communication device R6 as shown in FIG. 11, no communications can be made between the local communication terminal 40 connected to the remote communication device R4 and the local communication terminal 40 connected to the remote communication device R6 because of the troubled spot (marked X) as shown in FIG. 13.
Thus, the aforementioned conventional communications network system having the connection style accommodating a plurality of unit networks does not link neighboring unit networks physically. Therefore, if one unit network had a trouble such as a disconnection fault, alternate communications cannot be made via another unit network, and communications between the local communication terminals adjacent to the troubled spot are disabled, resulting in degrading the reliability of communications.
In view of the circumstances described above, it is an object of the present invention to provide a communications network system and a method for controlling its communication path, that the disadvantages described above are remedied and if a trouble occurred in a unit network, an alternate path running via another unit network can be formed to relieve communications between local communication terminals adjacent to the troubled spot, and reliability of communications can be improved.
The invention also aims to provide a communications network system and a method for controlling its communication path, which are effective to relieve communications in case of a trouble in any one of unit networks forming a bus type connection to connect respective communication devices to a path.
Besides, the invention aims to provide a communications network system and a method for controlling its communication path, which are suitable to relieve communications in case of a trouble in a unit network when a path connecting respective communication devices within the unit network comprises a circuit according to an ATM communication method.